This invention relates generally to the field of electrochemistry, and in particular to a filter press electrochemical cell design incorporating an improved fluid distribution system.
Historically, the design of electrochemical cells has been oriented toward the processes for which they were to be used. The result is that there are many cell designs of limited utility. For instance, the electrolysis of sodium chloride solutions has dominated industrial electrochemistry but the cells used for this purpose are not applicable to other processes, particularly organic electrochemistry. While a certain amount of technology crossover is possible, as a general rule, these prior art cells work only marginally well at best in processes for which the cells were not specifically designed.
The advances which have been made in the design of these process-oriented cells have evoked interest in designing cells which would be applicable to a variety of different processes, without making major concessions on process performance in any particular case. Examples of these advances which have given credibility to the idea of a truly multipurpose cell design are the development of stable ion exchange membranes, the commercial availability of improved electrode materials such as the dimensionally stable anode, the availability of improved plastics from which the electrically nonconductive parts of the cell can be made, and advances in electrochemical engineering science.
With regard to organic electrochemistry, cost and other factors of cell design development have often not justified process-oriented cells. For example, the gathering of engineering data, construction and testing of cell prototypes, and problems of achieving a successful cell whether process-oriented or multipurpose, have caused companies engaged in chemical production to be reluctant to launch major efforts in mechanical design engineering where adequate economic justification was not present. The best economic justification is, of course, with a large-volume product. Hence, there has been more reluctance and even more of a need for a multipurpose cell in the production of intermediate- and small-volume chemical products.
Most electrochemical cells regardless of design have several common components. These are a pair of electrodes corresponding to an anode and a cathode, a cell body or frame, and some type of separator if a divided cell is desired. Variations of this basic cell have included flat plate and capillary designs, packed bed and fluidized bed designs, and even pumped slurry electrode cells. Of these, the flat plate electrode cell is most common, and is typically used in a filter press arrangement composed of any number up to one hundred or more of individual cell units or compartments formed in a single unified cell bank.
Two types of fluid and current flow are possible in such a collection or bank of cells. One is series flow where the discharge of fluid from a preceding cell is routed to the inlet of the next cell, this routing being accomplished internally within the cell body or externally using pipes, conduits, tubes or manifolds. In a similar fashion, a series or bipolar flow of electrical charge (current) can be accomplished, for example, by connecting the anode of the preceding cell to the cathode of the next cell in either a galvanic or electrogenerative cell arrangement. The other type is parallel or monopolar charge flow, which routes fluid or current to the whole collection of cells at one time from a common source or supply. Again, this can be accomplished internally or externally of the cell framework.
The preferred mode of operating a cell bank with regard to this fluid and charge flow is determined to a large extent by the process involved. Parallel fluid flow is generally preferred for many electrochemical processes as is bipolar electrical connection. However, a notable disadvantage of bipolar connection is that a certain amount of current is conducted by the fluid in connecting manifolds. This so-called by-pass current reduces cell efficiency since this flow of charge does no useful chemical work. Preferred spacial orientations of a cell bank may also exist, especially where gases formed during a particular electrolysis cause losses in efficiency by increasing resistance to current flow.
Two important features of an electrochemical cell design are the electrode and the fluid distribution network. The usable surface area of the electrode determines in part the production rate of the cell, and thus a goal of cell design development is to create as much electrode surface area within as small a cell volume as possible without altering or disturbing the other parameters of cell operation in a detrimental way. Toward this end, electrodes composed of packed or fluidized particles, expanded metal mesh, and reticulated materials such as carbon "sponges" have been developed. Most of these enhanced-surface-area (ESA) electrodes are permeable to the fluid within the cell and give rise to two types of fluid flow within the cell compartment. The flow-through type routes fluid parallel to charge flow and has been accomplished in various cell designs including the filter-press type. The flow-by type routes fluid perpendicular to charge flow and has been accomplished particularly in cylindrical cell geometries, but has not been shown commercially viable in filter press cells to the applicant's knowledge.
Fewer advances have been made in the design of fluid distributors. Distributors employing both internal and external manifold systems have been used. The important consideration in such networks is to connect the manifold to the cell in such a way as to evenly distribute fluid from side to side and across the thickness of the electrode compartment in each cell. Little concern appears to be given to fluid distribution in the case of prior art series flow cell banks. In the case of parallel fluid connection between cells, there is the additional criterion of needing equal flow into each cell connected in parallel to avoid adverse effects caused by mass- and heat-transfer variations. Other advantages also exist with even inter-cell distribution and would be obvious to those skilled in the art.
The usual approach to fluid distribution has been to accomplish the task in as small a space as possible. Most fluid distributors consist of a multitude of channels or orifices which attempt, through various means, to equalize and distribute flow rates between them. A disadvantage of such designs is the tendency of these channels to become clogged with particulate matter thereby disrupting the even distrubition of fluid or, in some cases, actually stopping fluid flow altogether. Another disadvantage, especially for electrochemical cells, is the tendency of these small channels to prevent rapid release of bubbles in the fluid which can disrupt even fluid distribution as well as cause increased cell resistance to current flow. Still yet another disadvantage is that such short distribution networks tend to maximize by-pass current losses when bipolar electrical connection between cells is used. In commercial-sized cells, all of these considerations are quite important to the optimum and efficient operation of the electrolysis reaction.
These prior art distribution networks have also suffered from having restricted supply and discharge openings and other complicating barrier structures or other constrictions in the fluid path which render them more prone to clogging, gas blockage, maldistribution or channeling of fluid, and other maladies with attendant and unavoidable harmful effects. Such structures add complexity and increased design and maintenance costs to the cell, as do the lattices, grooves, or goffering often found on exposed electrode surfaces or within cell compartments to supposedly promote better fluid distribution.